Group communication refers to a communication mode in which a network transmits service data concurrently to a number of terminals in the group, and FIG. 1 illustrates a network architecture of group communication, where respective interfaces in the network architecture function as follows:
An M3 interface is configured to provide the function of managing a session between a Mobility Management Entity (MME) and a Multi-cell/Multicast Coordination Entity (MCE), e.g., notifications of starting a session, and stopping a session, etc.;
An M2 interface is configured to provide the functions of managing a session, and transferring Multimedia Broadcast Multicast Service (MBMS) scheduling information, etc., between the MCE and an evolved Node B (eNB);
An M1 interface is configured to provide an MBMS service transmission between an MBMS Gateway (GW) and the eNB;
A GC1 interface functions as an interface between a User Equipment (UE) (or referred to as a terminal) and a Group Communication Service Enabler Application Server (GCSE AS) to define service layer signaling for related functions of the GCSE, e.g., session setup, traffic control, etc.;
A GC2 interface is configured to provide interaction between a Broadcast Multicast-Service Center (BM-SC) and the GCSE AS;
An SGi-mb interface functions as a user plane interface between the BM-SC and the MBMS GW; and
An SGmb interface functions as a control plane interface between the BM-SC and the MBMS GW.
In the group communication mode, group communication terminals can be categorized by their roles in group communication into two types: transmitting terminals, which are terminals authorized by a group communication server to transmit group communication data to the group communication server, where uplink data between the transmitting terminals and the group communication server are transmitted in a unicast mode; and receiving terminals, which are authorized by the group communication server to receive group communication data issued by the group communication server, where the receiving terminals can receive group communication data from the network in a unicast or multicast mode.
For a group communication service of a Long Term Evolution (LTE) system, in a case that there are a small number of user equipments, then the network side (i.e., the GCSE-AS) can transmit a service over an LTE unicast Evolved Packet System (EPS) bearer; and in a case that there are a large number of user equipments, the network side can broadcast the group communication service over an existing MBMS network. In order to make full use of network resources, the network side can switch between multicast transmission and unicast transmission according to a preset strategy. For example, if there are a decreased number of user equipments receiving certain group communication service in a cell, then the network can switch the service from multicast transmission to unicast transmission in a related MBMS Single Frequency Network (MBSFN) domain.
However after the network switches the service from multicast transmission to unicast transmission in the related MBSFN domain, since no appropriate unicast EPS bearer has been set up for the user equipment, the network can not transmit the group communication service over any unicast EPS bearer, so that the group communication service may be interrupted for a long period of time while group communication is being switched from multicast transmission to unicast transmission, and as a result, service continuity required for group communication may not be accommodated.